The two defendants, both Britons of Nigerian descent who have asked to be identified by Islamic names in court, deny murder.

As the trial opened at London's Old Bailey court, prosecutor Richard Whittam said the pair had run Drummer Rigby over with a car, knocking him unconscious, before attacking him with knives and a meat cleaver in a "cowardly and callous murder".

Victim: Security footage of Drummer Lee Rigby at Woolwich railway station before he was killed. Photo: AP

Drummer Rigby's relatives watched from the courtroom as Mr Whittam told the jury of eight women and four men that Mr Adebowale had stabbed and cut the soldier while Mr Adebolajo tried to cut his head off.

They had dragged his body into the middle of the road, Mr Whittam said, because "they wanted members of the public to see the consequence of what can only be described as their barbarous acts".

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"He was repeatedly stabbed and it appears it was Michael Adebolajo, the first defendant, who made a serious and almost successful attempt to decapitate Lee Rigby with multiple blows to his neck made with the meat cleaver.

"At the same time as Michael Adebolajo used the meat cleaver, Michael Adebowale was using a knife to stab and cut at Lee Rigby's body."

Drummer Rigby's widow, Rebecca, walked out of the court in tears during the evidence.

There were gasps from the courtroom as the jury was shown security footage of a car veering onto the pavement and ramming into the soldier.

When the police arrived, Mr Adebolajo ran at a police car waving the meat cleaver and was shot by officers, the prosecutor said.

Mr Adebowale charged at police with a gun, and was also shot.

Mr Adebolajo and Mr Adebowale are further accused of attempting to murder a police officer and conspiracy to murder a police officer. They deny these charges.

The horrifying murder took place in the middle of the day as a group of schoolchildren were returning from a library, Mr Whittam told the jury. Members of the public turned them back to avoid the "awful" scene.

The prosecutor said passers-by had shown "bravery and decency" in the aftermath of the attack, with one woman confronting Mr Adebolajo even though he was still holding the meat cleaver and his hands were covered in blood.

Another woman "went to the lifeless body of Lee Rigby and stroked him to provide some comfort and humanity", Mr Whittam said.

Mr Adebolajo has asked to be referred to as Mujaahid Abu Hamza in court. Mr Adebowale wants to be called Ismail Ibn Abdullah.

Both defendants have admitted possession of a firearm with intent to cause violence.

Drummer Rigby, the father of a young son, had joined the British Army in 2006 and had served in Afghanistan.